I hate to break it to you, but no, there's no related _observable function that you could attach to in order to detect the cursor switching between sub columns.
I do understand that you want to have Paketti react to various things being done within Renoise, but there's quite a bunch of stuff that simply isn't very practical in this regard.
It's why we don't have notifiers for things like the current playback position, for example, simply because it would get spammed like hell and bog down the entire scripting engine.
For certain things you can therefore only manually check what the current state is, at some particular interval or after some particular event that occurs somewhere else in your tool.
yeah, i fully getcha - some notifiers would be total firehoses and both bog down the script and the app..